The Mini-Machine Flies With Apogee

13mm Motors

William Orvis, LUNAR#309

After reading about the new Apogee 13 mm composite motors in the November/December 1999 Lunar Clips, I just had to build a rocket to try them out. I guess I could have stuck one in an Estes Mosquito to try it out but I wanted to see the rocket on the way up and have some chance of finding it after the launch. A Mosquito would just disappear off the launch pad, never to be seen again. I decided to build a rocket that was a little bigger than the ones designed for the Estes T engines to see what this motor could do. My old Estes Mean Machine (the flying fence post) has had many crowd-pleasing flights, so I decided to design a mini version of it to try out these new engines.

The rocket I designed used three 16 inch BT-5 tubes, a nose cone and four small fins the same shape as those on the Mean Machine. The first problem I ran into is the lack of tubing couplers for BT-5 tubes. That was solved easily with a couple of expended T sized engine casings. I put the separation point for parachute ejection between the first and second engine tubes (see figure) instead of at the nose cone so that the mini engine’s ejection charge would not have to pressurize all three tubes. It turns out that would not have been a problem as the 13 mm composite engines have a strong ejection charge. I attached the shock cord using the slit tube technique. That is, I wove the shock cord through two slits made in the tube, put a knot in the end, and glued it in place.

The next problem occurred when I tried to install a recovery system. Have you ever tried to fit a parachute in a BT-5 tube? Even a 12 inch plastic chute is an extremely tight fit. I finally managed to fold one in such a way as to get it in, but that was a bad mistake. At ejection, the parachute was so tightly packed that it hung up on the shock cord attachment and didn’t deploy. It didn’t even get out of the tube; I should have used a streamer.

The Apogee 13 mm composite motors come in B7 and C6 sizes with delay times from 4 to 10 seconds as shown in the table below. The table was taken directly from the Apogee web site (http://www.apogeerockets.com). While Apogee lists engines with delays from 4 to 10 seconds, at the time I purchased engines the only models available were those with 4 and 8 second delays. The motors available at the web site seem to change from week to week so you can probably get the delay you want if you aren’t in a hurry. These motors are also a little pricey compared to the Estes T engines but if you want a lot of lower in a small package, this is the only way to go.

* Apogee suggests you do a simulation for these engines to determine the maximum launch weight as they are a long burn lower thrust motor than a standard C motor.

While both the B and C motors are 13 mm in diameter, the B motor is 51 mm long and the C motor is 83 mm long. Compare this to the Estes T motors which are 44 mm long. You could probably put a B motor in a rocket designed for an Estes T motor and not have any problems as it only hangs down a half cm lower than the Estes motor. To use a C motor in a rocket designed for the Estes T motors, you would have to remove the engine block. I built the Mini-Machine without an engine block so it could handle either of the Apogee motors or an Estes T motor. At flight time, you must wrap the bottom ¼ inch of the motor with masking tape to build up a lip to prevent the motor from sliding up inside the rocket tube. This technique is familiar to high-power rocketeers, but is not common in low-power rocketry.

On February 19, the rain stopped and it turned into a beautiful day for flying rockets. There was a little wind later in the day, but in all it was a great day for flying. I pulled out my Mini-Machine with its fresh coat of bright red paint and began prepping it for flight. The first problem occurred when I tried to install the engine. It just didn’t want to go in the BT-5 tube. Trying to force it in started to split the paint job. While the engine is 13 mm in diameter, Apogee wraps a nice little sticker around the engine with their name and the engine type printed on it. That extra thickness made the engine too tight to fit in the engine tube. I solved this problem by simply pealing off the sticker. Luckily, all the adhesive stayed with the label so I did not have any problem with the engine sticking in the body tube.

I chose the B7-4 engine for this rocket, as I did want to get my rocket back and the wind was blowing some of the rockets pretty far down range. I wrapped the bottom ¼ inch of the engine with enough layers of masking tape to prevent it from sliding up into the engine tube, inserted the engine, and taped it in place.

The next problem involved getting the included Copperhead igniter into the throat of the engine. As you might expect, the throat of a 13 mm engine is small and a Copperhead igniter will not go in unless you hit the hole just right. It took several minutes of feeling around with the igniter until I finally found the hole and slid the igniter deep into the engine. When you first insert the igniter, it goes about a half inch into the engine. This is not where you want to be. There is a small hole that you have to find with the igniter. When you find the hole and hit it at the right angle, the igniter penetrates more than an inch into the engine. This is where you want to be.

Ready to go, I took my new toy out to the pad and hooked it up. Everyone wanted to see what this long red needle would do. The LCO pushed the button, the Copperhead made a satisfying pop and the rocket just sat there. These engines are really hard to light. Two more Copperheads and some colorful language later and it was still sitting on the pad.

At that point, I made a visit to the Unique Rocketry table and bought an IgniterMan igniter. The IgniterMan igniter slipped easily into the engine with no more than 2 seconds of feeling around to find the hole. I put it back on the pad, the LCO did the countdown, and away it went like a bright red needle slicing into the sky. I’m sold; I went back later and bought myself an IgniterMan kit.

It had a beautiful flight with the ejection charge firing right at apogee but then the problem with the parachute occurred. The rocket separated as it was supposed to but the tightly packed parachute did not eject. The ejection charge just packed the parachute tighter instead of popping it out. The pressure eventually tore the masking tape and ejected the engine. There were cracks in the paint below the parachute indicating that the engine pressurized the tube almost to the point of bursting. The rocket tumbled to the ground, but was light enough that it was not damaged. It was actually kind of lucky that the parachute did not work, as the wind would have carried the rocket into the housing south west of the launch site. This rocket should do just fine on a streamer or two.

Being based on experience with a single engine, this conclusion is in no way statistically sound but these 13 mm composite engines appear to be really hard to light. Copperhead igniters just couldn’t do the job but the IgniterMan igniter lit it off the first try. I can think of a lot of interesting configurations for these engines, for getting small rockets really high or for fitting an engine into a small place on a larger rocket. As to the Mini-Machine, it will fly again but I want to try the C engine this time. Of course that would be on a day with no wind, streamer recovery, and my kids along to chase it.